This invention generally relates to processes for producing lead storage batteries with a thixotropic gel electrolyte comprised of sulfuric acid and a gelling agent.
Essentially, there are two methods for introducing fixed sulfuric acid electrolyte into lead storage batteries. In a direct filling method, a freshly prepared thixotropic mixture of sulfuric acid, having a specific gravity which conforms to that of a normal filling acid, is combined with finely dispersed silicic acid as the gelling agent. In a two-step filling method, only the electrode plates together with the separators are first impregnated with a partial electrolyte, which is comprised of sulfuric acid without a gelling agent, and thereafter, free electrolyte space is filled with a phase of the electrolyte that exists as a thixotropic gel.
The problem with the direct filling method is that a sol-gel transformation of the introduced mixture immediately starts to occur. This gives rise to the danger that the pore openings in the plates will become clogged before the electrolyte has penetrated to the interior. This danger can be only partially eliminated by the application of a vacuum. What is more, there is still the disadvantage that the pore system of the plates and of the separators will be adversely affected by solidification of the sulfuric acid that has been absorbed.
The present invention is directed to the two-step filling method. Previously known variations of this method differ only with respect to the composition of the acid and gelling agent, the acid concentration, and the addition of intermediate partial charging and discharging steps. However, what these methods have in common is that there is a separate introduction of sulfuric acid containing no gelling agent into the plate and separator pores (by impregnation in an excess amount and removal of the excess) in a first step, followed by final filling of the storage battery case with a thixotropic filling electrolyte in a second step.
DE-PS 3,041,953 discloses that a well-defined amount cf sulfuric acid can be electrochemically bound in the pores of the active compounds of the electrodes by filling the storage battery with a liquid sulfuric acid electrolyte and then discharging the battery. The remaining, freely mobile liquid electrolyte is then removed (by pouring it out). This removed electrolyte can then be used as a filling electrolyte after it has been mixed with a gelling agent. Once the storage battery has been filled with this mixture, it must then be brought into a state of operational readiness by recharging, which at the same time initiates the gelling process. However, this requires inversion of the cells for removal of the freely mobile electrolyte, and the required electrical treatments are time-consuming.
DE-PS 3,521,200 describes a filling process that avoids extra discharging/charging steps. However, this leaves the disadvantage that the cells must still be inverted during emptying. As a result of this, the cells are contaminated with acid. What is more, the filling of batteries that have already been connected is made more difficult, and the production process is retarded.